Leyo Motorsport Billet Aluminum Trim Rings
Before
After
The quality of the trim rings is just as impressive as the packaging, they are knurled aluminum, have a nice tactile sensation, and are held in by o-rings securely to your existing knobs. No permanent modification or adhesives necessary. They also compliment the Audi 12V plug that many people fit to their cars. My only complaint might be that the smaller rings appear to stick out further than the larger ones, but I think it's just an illusion as the overhang is not as noticeable on the larger rings.
Recommended for that upscale feel!